text:
Is it hard to hover a coaxial rotor helicopter?
Convetional rotorcrafts are hard to hover as when we increase collective, even tail rotor pitch needs to be compensated. But in coaxial design, will just increasing the collective do the job as torque doesn't change?

The difficulty in hovering a helicopter lies in the fact that hover is unstable. Any tiny divergence will increase over time, so you have to keep adjusting the controls all the time. And this does not differ between single rotor with anti-torque and dual-rotor.

I have no experience flying coaxial rotors, but I wonder if they are as difficult to hover as single rotor. The difficulty with single rotor is that a change of torque requires a change in anti-torque pedals, which will induce a side force that moves you off your position. This is then countered by opposite cyclic, which will stop the side-way motion, but now you have changed your power demand, so you will either slowly climb or descent. So then you change collective, but that changes the demand in torque and now you are full circle. I don't think co-axial hover has that much cross coupling

@DeltaLima, I don't have personal experience either. There shouldn't be basically any cross-coupling, but I wouldn't expect it to be that much of an issue—basically I expect that with a bit of experience, adding a bit of bank and yaw together with cyclic should become muscle memory, and the ratio is always the same.

In the multi-rotor (be it coaxial or tandem), if the torques are not balanced the fuselage will spin round. This is basically the same problem as the single-main-plus-tail-rotor.

@GuyInchbald balancing of the torques is done with the torque pedals. But in multi-rotor the torques are always balanced with the pedals centered, because torque of both rotors changes the same with collective, while in single-rotor the torque of main rotor changes with collective and the torque of the anti-torque rotor does not, so the pilot has to add a bit.

There is more than just compensating the tail rotor pitch (which I believe is somewhat compensated for anyways mechanically), which makes hovering a single-rotor helicopter challenging. You also need to tilt the main-rotor to compensate for the side thrust of the tail. If the main rotor is vertical you drift sideways.
However, keep in minds rotors are never perfectly identical. So you still need to compensate manually, if even a very small (possibly unconscious) amount.
I don't know if it's hard on a coaxial, but it should definitely be easier than on a single-rotor.

I don't think there should be much difference, really. Both are unstable in hover, and similarly sensitive. That's what matters.

@JanHudec In the world of model helicopters at least, coaxial helicopters are known as much easier to hover and fly than single rotors because they are more stable. A larger significantly single rotor is still significantly more squirrely than a smaller coaxial.

The model controller is both less accurate and harder to manipulate in coordinated fashion, so I'd expect the cross-coupling to be much bigger issue there.


text:
Can coaxial rotors be mounted on a piston engine like the Lycoming O-320?
What are the drawbacks of this compared to coaxial configuration with a turboshaft engine?

Even on piston engine helicopters the rotors are not directly connected to the engine, they go through a transmission. I don't see why you couldn't, although the O-320 probably wouldn't have enough power to turn two full-size rotors.

It is theoretically possible to mount coaxial helicopter rotors on any engine, but it would not be economically feasible on a small engine like an O-320. Turboshaft engines typically have much more power than an O-320. Why would you compare an O-320 engine to a turboshaft engine?

What the hell... Once again a totally legit question here, and someone gives it a minus one. Come on guys/gals... at least have the courtesy of commenting why you think this is a bad question!

Stan Hiller's earliest helo designs were coaxial and used relatively small 4-cylinder aircraft engines to drive the coax transmission, so it is indeed possible to power a small coaxial helicopter with a relatively small piston engine.
Hiller also found that autorotating a coax main rotor blade setup was difficult and so his later designs used a more conventional single main rotor blade and a tail rotor.

That's interesting about the autorotation problems.  Must've been related to the mechanism he was using to get differential torque for yaw control?

do not know... -NN

Sure. Earliest Kamov production helos - Ka-8 and Ka-10 - were powered by piston engines.
https://en.wikipedia.org/wiki/Kamov_Ka-8

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

sort: votes


text:
On an average, space of how many economy class seats is taken up by a suite?
The suites offered by many airlines for the super rich, are very spacious and convenient. Though, I can only Read about them! One thing I can see is, the A380 is capable of upto 853 economy only passengers, but most of them carry 500 economy, 65 business and 12 suites. So, they must be eating some considerable amount of space. What I am asking is, basically the number of economy class seats that would fit inside a first class cabin and not how much space is available for first class and economy class cabins.

[If this is not a duplicate...] I'm in favor of moving it to Travel:SE, as suggested in the possible duplicate by Pondlife.

The super rich travel in their own jets, you just mean regular, not filthy rich.

The size of the seat is slightly different in each airline, but according to this table, 32"×18" is the smallest, and most common, economy seat. That is 4 ft².
According to the article you linked, the largest suite is 125 ft². That is 31¼ times more.
Remember, however, that space is not the only consideration. The main consideration for aircraft is weight and obviously first class passenger does not weigh more than economy class one.
Now consider that those 853 passengers, with baggage, will weight around 80 t. Maximum take-off weight is 575 t of which 276 t is empty weight and the tanks can take about 260 t of fuel. But that leaves you only 39 t for payload with full tanks! So on very long routes, the aircraft can't pack 853 passengers anyway. The 500 is about maximum that makes sense (for long range variant, which is most common).
So they don't mind wasting some space for luxurious first class if they can charge premium for it.

"and obviously first class passenger does not weigh more than economy class one." the suite itself probablly does weigh more than an economy class seat though.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
Can anyone identify what appears to be a crashed WWII German plane?
This photograph is in my family's photo collection.  The only information with the photo is, "Cousin Bud Kohl - aircraft wreck."   If anyone can identify the make and model it is greatly appreciated.

Although we can't see any markings, it is also possible the P-47 was one of those captured and flown by the Luftwaffe, or cannibalized to make the other captured aircraft flyable again, hence why it would be with parts of German airplanes.

It's a P-47 Thunderbolt, the original "razorback" version (not the later ones with the bubble canopy and cut-down rear fuselage).  The instant giveaway is the V shaped windshield, which had the armour glass as a separate panel behind it (removed in the pic). The bubble canopy versions have the armour glass as the windshield itself, like most other fighters.
The German rudder/fin looks to be a piece of a the right tail boom of a Focke-Wulf FW 189.
My guess is that Bud is probably at a forward P-47 base that was an ex-Luftwaffe base (the brick wall behind suggests a large base with permanent buildings, possibly within Germany or Eastern France, in early 1945) and therefore still strewn with German wrecks.  The Jug in the picture has had what looks to have been a gear up landing and was dragged away to a bone yard area, and is being cannibalized for parts as required.

This is incredible information for me, and so detailed.  Thank you very much.  I never would have guessed a P-47 due to the German tail and wing in the pile. And the suggestion of a belly landing looks spot on due to the propeller damage.  This information is GREATLY appreciated.

Thanks.  The brick wall there is quite distinctive, so if you search around on the web for "P-47 bases in Europe" or go to different P-47 squadron historical website pages, you might stumble on to a photo with that same building that nails down the precise location.  A little time rooting around may present a Eureka! moment.

I've heard that some P-47's were captured and flown by the Luftwaffe.

Yes there were and if you hunt around you'll find at least one pic of a Jug in a hangar with German markings. But I would say the scenario I described is more likely in the circumstances.

This looks like a P-47 (twin row radial, 4-blade prop, unique canopy frame structure) that is part of a scrap pile with some German types in the mix.

Thank you, Niels.  You confirm the comment above.  I never would have guessed it to be a P-47.  Much appreciated.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
Can anyone identify the Make and Model of this nosed-over plane?
I've tried to identify this plane by the writing on the plate on the cowling but have been unable to match the logo, and when I've blown the size up the writing is unreadable. It looks something like SCOOTROM with a long tail on the upper part of the "S" and with two straight lines under it.  I think this light crash took place in Wisconsin, perhaps Madison or Stoughton but I don't have a year.

I believe this is a Stinson HW-75 (Also called the 105 Voyager).
I know for sure this is a Stinson aircraft due to the logo which was often rendered on the side of aircraft with a double underline and certainly has the long tail on the upper part of the S as you described:

Logo from the orignal picture, rotated and de-skewed
The identifying features which make me think this is an HW-75/105 Voyager are:

Similar decals down the side of the fueselage as shown in image below
Twin Supports meeting at the main gear roots
Shape of the step for access to the cockpit
Grill both side and below the spinner

source: Wikimedia commons
However there is a slight chance that this is actually another Stinson model such as a 108 or L-5 Sentinal which both share some similarities

Damn it, you beat me to it :-)

@DeltaLima It had to happen eventually. I'm an amateur though - do you agree its a 105 and not a 108 or L-5? (And thanks for the improvement showing the logos side by side!)

I am pretty sure it is a 105. The 108 is very similar, but the decals on the side of the 108 were different. I have seen pictures of the 105 with exactly the same decals as the crashed aircraft.

No question it's a 105 in my opinion.  The straight leading edged, fabric-covered fin confirms it.  The 108 had the metal fin with the distinctive external ribs (corrugations). Also the stabilizer location is lower than the 108.  Also the nose bowl is from the 105. The 108 had a different grille arrangement and a box projection below for the airbox.  It's not an L-5 either; no greenhouse.  I thought the flipped one might have an Aeromatic prop, but the logo on the blade is wrong.  Possibly just a ground adjustable wood prop of some kind.

You guys blow me away.  I had done the same thing with the logo, blew it up and flipped it over, and my result looks the same as the one posted, I guess by Jamiec?  And seeing it next to that Stinson logo nails it as a Stinson.  I swear, I spent over an hour trying to identify this plane, as I do with all my photos, but at some point, I give up and come to you experts.  I'm amazed, and pleased, that so far every photo I've posted, you've come through for me.  Please know that I do try, and have been able to identify the planes in many of my other photos, but when I dead end, here I come. :-)

@FreshBreeze It's fun to try to identify them. I have to be honest I have no special ability other than I started looking at pictures of vintage monoplanes, looking specifically for that double strut, which is a fairly uncommon feature.  It was only after I found a few that looked likely that I back traced the logo. ie, I identified the plane by looking at pictures before I put 2 and 2 together to get the logo match.  It was DeltaLima who produced the deskewed image from your picture and matched it up with an image I had added to this question of the logo

@Jamiec, I did the same thing.  I even did a search of aircraft manufacturer logos. I'm just not very good at it. I'll type ten or more different descriptions of a plane and still come up empty.  Well, not always.  Actually, I've been able to identify at least half of the plane photos I'm going through.  But I get totally stuck and frustrated with the planes I post here and I'm extremely happy that you, DeltaLima, and a few others have so far been 100% successful in helping me.  I can't express how much I appreciate this.  When I get all my photos labeled I'm going to share a YouTube video.


text:
How does a 'Highly Swept leading-edge' delta wing reduces fuel burn and increases range?
Spike Aerospace  claims to have designed an aircraft called S-512, with maximum speed of 1.8 mach and cruise speed of 1.6 mach, speaking about the notable changes in the aircraft, Spike Aerospace says:

“The new delta wing of the S-512 delivers high aerodynamic efficiency and improved flight performance in both low-speed flight and supersonic cruise,” “The highly swept leading-edge reduces wave drag and consequently reduces fuel burn and increases range.

How does a highly swept leading-edge helps?

Spike aerospace has not built anything, they have a concept.

Concorde was not a faliure because of technology. It failed because of loss of consumer trust after the 2000 accident (which was some other airplane's fault) and decreasing passengers after 9/11

Spike Aerospace have 1-10 employees, operate out of office suite 304 at 349 Newbury St, Boston and, so far as I can tell, have no manufacturing facilities or history. I don't think there's any way to be sure Spike will ever build even one S-512, let alone support them in commercial operation for over a quarter of a century.

While they have just a concept, @RedGrittyBrick, I'm glad they have that. If Orville and Wilbur hadn't dreamed despite their father's protests, we might still be driving NYC to LA...

@VictorJuliet: These are not the only reasons for ending Concorde commercial adventure. Concorde was not able to continue due to the pollution, noise, and once revolutionary but now old technical design, which was a factor in the 2000 accident, not mentioning the ticket price (the customers were able to afford it and happy to pay anyway). 9/11 did not slow down air traffic, which is increasing.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
Are military and jet pilots given any extra training before entering commercial aviation if yes, then how?
Because a jet pilot is habitual of flying super sonic aircraft, which can possibly go in any direction and do many maneuvers, which commercial aircraft can't, are they given any special training before entering commercial aviation?

Of course a pilot needs to be trained and rated for the aircraft that they are going to fly. You can't take an F-16 pilot and drop them in a 787 and expect that they will conduct a safe flight. They need to go through as much training as non-military pilots, save for a few that have time in type (like Air Force 1 pilots)... The biggest difference is that military flight time counts towards ratings and commercial/ATP.

Its also important to note that a military pilot is not an FAA certificated pilot, in order for that to happen you have to go through a conversion process.

Apart from flying the aircraft itself, there are a lot of FAA regulations and the airline's own operations rules to learn too. Even a pilot who moves from one airline to another would need some training.

@RonBeyer @Pondlife; your comments are valid but please note that the FAA only regulates aviation in the USA, not in the rest of the world. The OP is not from FAA land.

@DeltaLima Very true, but if you remove "FAA" from my comment I think the point is still valid, regardless of who the regulator(s) involved are.

@anshabhi The question as asked is very broad  and basically requires answers to explain all of the training for type ratings and line checks.  You would definitely learn more if you did a small amount of research ahead of time and asked questions about what you don't understand.  As I recall, Cockpit Confidential has almost an entire chapter on commercial pilot training.

@CodyP hmm yeah.. But a consice answer should be good enough..

@anshabhi Its difficult to give a concise answer without a focused question. Could you at the very least narrow it down to a country?

@RonBeyer let's take USA then.. (Sorry for late reply)

I had a friend who was a Marine C-130 pilot, when I first got my CFI.  I took him up in a Cessna 172.  It was really interesting because his judgements were way off, but I only had to correct him once for ANYTHING.  He tried to fly the approach at 110 kts (one comment and it was fixed).  He tried to flare at 150 ft (show him once and it was fixed).  I find that a good and experience pilot can very quickly be trained to fly a different aircraft.

Watch out for the words "centerline thrust only"!
The overall answer to your question depends on the military pilot's background and experience.  Military heavy jet pilots (bomber and transport) will have very little difficulty converting over to civil airliners, as civil airliners and military transports of equivalent role are close cousins in flight characteristics, and many bombers aren't that far off either.
Fast-jet pilots, however, can run into one specific difficulty when transferring to civil jet aircraft.  Most (albeit not all) twin-engine tactical jets have their engines mounted very close to the aircraft centerline.  This means that there is very little yaw when an engine fails in a tactical jet, which is quite difference from the response of an airliner, or even most light twins, during an engine failure.
Hence, a pilot whose sole multi-engine experience is in twin-engine tactical jets may receive a "centerline thrust only" restriction on the CPL/AMEL certificate they are issued on the basis of their military experience.  This restriction must be lifted before they can fly or receive a type rating in a non-centerline-thrust aircraft (even a DC-9 yaws enough on engine failure not to count).  Thankfully, it's not a hard restriction to lift -- in the worst case, it's simply a matter of passing the multi-engine checkride in a light twin with wing-mounted engines, and may be liftable as part of a type rating course for a non-centerline-thrust multi-engine aircraft for that matter.

Note: this answer is based on googling, I am not a pilot.
Military pilots in the USA can apply for some civilian aviation qualifications on the basis of their military qualifications and experiance.
https://www.law.cornell.edu/cfr/text/14/61.73
Sepcifically these include
Commercial pilots licenses.
Instrument ratings
Type ratings
Flight instructor certificates (for those who have been flight instructors in the military)
Whether those qualifications will be sufficient will depend on what they want to do. Many larger commercial operations will require type ratings (which can be converted but only if the planes match up) and Airline transport pilot licenses.


text:
Why are the call signs of some airlines completely different than their name?
The call sign of U.S. Airways is "Cactus", and that of British Airways is "Speedbird"; while for Malaysia Airlines is "Malaysia", Qantas is just simple "Qantas".
Why is it that the call signs of some airlines are so different from their names?

How do I make this question less broad? Help!

Looks alright to me.

Related question

British Airways uses "SHUTTLE" instead of "SPEEDBIRD" on its domestic flights. Back in the old days when BA had fast planes, those planes would use the call sign "SPEEDBIRD CONCORDE". For a while BA toyed with using "GHERKIN" on their LCY-JFK flights which were under a different company as part of a complex financing arrangement, but in the end they stuck with "SPEEDBIRD". So there is some scope for the company to have a bit of fun!

I am in a fix now, which answer should I mark as accepted? The most upvoted one, or some other..??

Whichever you feel answers the question best. Of course you could put it to a vote, but the results of that are already in :)

Cactus was from America West Airlines (desert has lots of cacti)

I found this on Wikipedia:

Some call signs are less obviously associated with a particular airline than others. This might be for historic reasons (South African Airways uses the callsign "Springbok", hearkening back to the airline's old livery which featured a springbok), or possibly to avoid confusion with a call sign used by an established airline. Companies' assigned names may change as a result of mergers, acquisitions, or change in company name or status; British Airways uses BOAC's old callsign ("Speedbird"), as British Airways was formed by a merger of BOAC and British European Airways. Country names can also change over time and new call signs may be agreed in substitution for traditional ones.

Before America West bought US Airways, their callsigns were Cactus (associated with the deserts of the American West) and USAir. After the merger, the combined airline kept the better-known (airline) name, and the other airline's callsign. Now, after the acquisition/merger of American Airlines, they're using that airline's name and callsign.  Leading to more than a few radio calls recently (late spring / early summer 2015) along the lines of

"Center, Cactuuu...merican 123, FL 350"

Posting it as an answer, since it was too long for a comment.

That last bit is funny, thanks. +1

This reminds me of this video where a Philadelphia controller was taking suggestions to improve the callsign of Air Wisconsin.

I will miss hearing Cactus

More reasons call signs might not be the name of the airline would include:

You need to have a call sign that contains few syllables rather than many, and that comes across relatively distinctly when communications are bad. The radio traffic can be fast, furious, and filled with static.
You need to have a call sign that doesn't expose you to ridicule. For example, when UPS first started flying with their own aircraft (as opposed to using contract aircraft and crews, which they first did), they used the call sign "brown tail." That didn't last long.
You need to have a call sign that does not give you political problems. For example, I worked for an airline that was known to be owned by Jewish interests and whose major route was New York to Tel Aviv. We also did a lot of charters. There was no way we could get overfly permits from either Iran or Saudi Arabia using our usual call sign, so we had an alternate, little used call sign for those flights.
You may be operating a contract flight. I flew many such flights, and the call signs I can offhand remember using included: Speedbird, Air India, Garuda, United, Japan Air, and Delta. Also on military charters we often used a military call sign, but I can't remember what it was. It was set up especially for the military airlift for Operation Desert Shield, the buildup for the first Gulf war.

Did the charters fly with "MAC" before the Military Airlift Command became Air Mobility Command (a no-longer pronounceable acronym), which then got "Reach" (as in, Global Reach, Global Power -- an Air Force slogan of the month for a while) as their callsign?

@RalphJ We normally flew military charters under our company call sign, but during Operation Desert Shield they had us use the military call sign (which I can't remember, but "Reach" sounds familiar). As I recall, the problem was that during the political coalition building, some countries not part of the coalition did not want aircraft supporting the effort in their airspace. Thus the call sign for the effort facilitated routing which made everybody happy. Switzerland never did let us overfly. By the way, before MAC, they used MATS, Military Air Transport Service.

Reach is used by Reach air ambulance in the SF Bay Area

Tower Air used TeeAir. Can you imagine the confusion if they had used Tower?

The callsign is the combination of the ICAO Radio Telephony (R/T) designator and a flight specific identifier consisting of numbers and letters. In addition to the R/T designator there is also a three letter designator used on flight plans and air traffic control display systems.
The R/T designator (callsign) must not cause confusion with other operators flying in the same area, preferably at most three syllables long and it must be pronounceable in at least one of the following languages; English, French, Spanish or Russian.
Since there are many operators that have "airline" or "airways" in their name, that word is usually avoided in the R/T designator as it easily causes confusion. Usually an alternative is found that is in some way related to the airline.
For pilots it is not too difficult, they are typically paying attention specifically to their own R/T callsign. For controllers it is more difficult since they have to associate the three letter code on their screen with R/T designators that are not related in an obvious way.


text:
Why is the A320neo considerably ahead of B737Max?
The A320neo has a 16% higher market share, 1022 more orders, 316 more MoU's and $122.1 bn greater estimated value than the B737Max.
Though both were released at almost the same time (231 days gap) with identical features and price (rather B737Max is  $15 mn cheaper), why is the A320 flying past the B737?
Data source : PDXLight

It is not for efficient reasons it is for, which aircraft that they can get the soonest that has than the previous model like the a320NEO has an efficient advantage over the a320

Sorry for the use of very agressive (and misleading) language earlier..

What sorry I was at school today so I have no idea what happened

The chat room Go ahead I join me I am very curious to know what happened

@Ethan nothing much happened..!! I just used words like lose out heavily to and race ahead of which were little inappropriate that's it... The sorry is for the community and not you specifically.. (Remember the @replies system)

I certainly would not say that the 737 MAX is "losing heavily" to the A320neo. The A320neo is entering service in the next couple of months (October 2015.) The 737 MAX isn't entering service for another 2 years (2017.) I don't think many aircraft manufacturers would be upset with having 2,727 orders for an aircraft that is still 2 years from even entering service. It should further be noted that the A320neo was right around the same total when it was 2 years from entering service.
Some additional factors that come into play:

The A320 line tends to be more popular in the low-budget Asian market, which is growing significantly right now.
The U.S. Dollar is currently relatively strong compared to the Euro, due to lots of economic problems in the Eurozone. This means that products exported by European manufacturers are relatively cheaper than products exported by American manufacturers compared to their usual price difference. Obviously, that's an advantage for Airbus, as it means that it's cheaper than normal right now for customers to buy Airbus vs. buying Boeing. This is, again, especially important to the cost-sensitive low-budget markets.
Airbus' Chinese assembly line allows them to make A320s for the Asian market quite a bit cheaper than Boeing can make 737s for that market, due to the major differences in labor costs.

It also probably depends on recent deliveries of and orders for the newer 737 models. Airlines that just upgraded are unlikely to place huge orders again for the max.

The 231 days gap is a big factor. Some airlines will want their planes sooner rather than later, which means the first one to the market will have an advantage. There is more than 231 days of gap between their entries into service. So if airlines are ordering planes out to, say, 2020, then naturally the type introduced earlier will have more orders. A graph from the linked source shows the number of orders based on days since launch. The A320 still comes out ahead of the 737, but the difference is not as dramatic.
Although the list prices are similar, no one will actually pay those prices. Both Boeing and Airbus will negotiate heavily with their customers to secure sales at much lower prices. The prices actually agreed to are typically not published, but it's possible that one factor is that Airbus is willing to give more of a discount.
Features are also not really identical. They may be very similar on the outside, but each manufacturer will tell you about the features that distinguish their product from the competition.
Another important factor is where the orders are coming from. Asia is a rapidly growing market, and the map shows that Airbus has received far more orders there. Airbus has an assembly line in China, which could be another factor working in their favor.

in Asia, India has developed some special love for the A320's. Indigo has ordered 430 of them,  two new airlines went with A320 and now boeing is keenly looking over to some order from spicejet..

One major factor that also plays into pricing is relative currency values. Right now, the Euro is down relative to the dollar compared to where it has been in recent years, so that actually works against Boeing and in favor of Airbus (because it makes Airbus' products end up being cheaper for export.) This is actually a rather big factor for international markets.

Airbus sells in USD, so it just makes more profits for them.  Anyway, for expensive purchases, currencies are not bought on market value but via constant prices through insurance and banks. Moreover, the euro drop is just a few month old, clearly not enough to explain the difference.

@Antzi Wider profit margin on the lists prices also means more room for negotiating lower prices, though, which can indeed help them to have more competitive bids. While you're right that the most dramatic drop-off has been within the past year, the EUR has been dropping against the USD since the recession started in mid 2008. I agree that this doesn't explain the whole difference, but it does make a difference.


text:
What are the difficulties and dangers faced by low flying aircraft?
Aircraft could fly low for various reasons,  Common reasons for low flight are sightseeing, photography, crop dusting, staying below Bravo/Charlie airspaces, flybys, and "just cause I felt like it".  What are the  difficulties and dangers faced by such aircraft?

What types of planes are you interested in? Military, civilian? This seems like a very broad question.

Have you read through any AOPA Nall Reports? Plenty of reasons in them.

The most common reason for low flight is to avoid RADAR?  LMFAO!! You've been watching too many action movies.  Common reasons for low flight are sightseeing, photography, crop dusting, staying below Bravo/Charlie airspaces, flybys, and "just cause I felt like it".  Avoiding RADAR is an extremely rare example.

@abelenky Thanks for it...

A lot depends on how low you are and what aircraft you're flying, but any low-level flight has some risks and difficulties. Here are some, I'm sure there are others:

If the aircraft loses power you have much less time and distance available to execute a forced landing, and your choice of landing sites will be more limited
You can't see as far ahead so it's more difficult to look out for obstacles and pilotage may also be more difficult in some circumstances
You may accidentally break the law on minimum safe flying height because the terrain changes too quickly, you don't see an obstacle or town until it's too late etc.
There's a greater risk of collision with terrain or obstacles like radio towers, especially if your aircraft is a fast one and you have less time to react
The likelihood of both thermal and mechanical turbulence is higher when flying low, increasing risks of loss of control and collision with terrain and obstacles
If you're scud-running in VMC there's a high risk of entering IMC accidentally, which is extremely dangerous (incidentally, despite the title of that question, even instrument-rated pilots have a poor safety record when they enter IMC unintentionally)
Radio reception may be poor
ATC radar may not be able to see you
Your engine may not operate very efficiently

That's not to say that low-level flying is always a bad idea, but it I think it does require more preparation, alertness and even local knowledge from the pilot.

I'd add that the risk of bird strike is higher at low altitudes.

CFIT is by far and away, the highest risk in the low level environment, and particular interest should be given to the time of day when making such flights as terrain masking could mean you impact a smaller feature masked by a larger one's shadow.  I'm leaving this as a comment because your answer is detailed and correct, I just wanted to stress one area for completeness.

TL:DR - basically terrain and birds.

Some of this depends on what you are flying as well, but as a rotorcraft pilot, we tend to fly at a lower altitude than planes. The number one killer of helicopter pilots is wire strikes. Wires, although they may seem obvious when you're on the ground, are next to impossible to see from the air. They're incredibly difficult to see both because the background is now the ground instead of the sky, and because you are likely to be quite a distance away (and if you aren't, its probably too late to do anything about it. You're either going to hit it, or not). You're also going to get quite a bit of mechanical turbulence, meaning turbulence that is caused by the wind blowing against things on the ground. Buildings, for example, will have air blowing upwards on one side of the building, and air blowing downwards on the opposite side. This air turbulence can extend quite a distance up from the object itself.  This will be true for buildings, hills, mountains, etc. In the case of a plane, you're also going to get reduced performance (in most cases) because the air is denser, and you'll have higher drag. Helicopters have the opposite effect, where they become more efficient with higher air density, so they'll fly better near the ground and at low altitudes.


text:
How do airlines manage routes when respective aircraft undergo B,C or D checks?
While playing some airline management simulation games, I found all these checks to be some real kind of mess. If suppose I have a fleet of about 10 A-320s then if one them goes for any check, then I would end up losing about 10% of my earnings. How do real world airlines make up for these absent aircraft? Like, consider an A380. It takes more than 2 weeks for a C Check to complete (>1 month for a D-Check), and it certainly translates into a big financial loss. How do airlines operate such routes routes in the mean time?

spare aircraft that get shuffled around

Airlines can also rent aircafts.

Essentially they plan on some fraction of the fleet undergoing some sort of maintenance check at any one time, so they build their flying schedule around the remaining fraction.  And then there is some amount of flex due to things needing unscheduled maintenance or coming out of maintenance early.
What makes the task easier is the fact that maintenance needs can be forecast fairly far in advance, and so the schedules can be built around a knowledge of how many aircraft are likely to be available.
As an illustration (not necessarily representative of any particular airline's actual numbers), let's say you have a fleet of 200 aircraft, and you know that on average each aircraft will be down for two 2-week heavy maintenance checks each year.  That works out to essentially one month out of 12, each aircraft is down, or a little under 10% of the time.  Add in a little time for shorter maintenance visits (but still longer than an overnight), plus an allowance for the fact that some aircraft just need unscheduled work, and the maintenance planners may end up telling you to plan on 175 aircraft available each day.  So that's what you build your schedule around.
Now let's say that one aircraft takes a birdstrike & an engine needs to be changed.  That is now a hole in the schedule.  It can be covered by cancelling a flight (undesirable, but always an option).  If an aircraft is finished with maintenance ahead of schedule (say it was scheduled down all day for some work, but the work is actually done by noon), that aircraft may be able to be put into service.  Perhaps it's possible to delay some sort of minor maintenance a day (not a heavy check, but something quicker) and essentially create one aircraft-day of capacity, and push the check off to a weekend day when the schedule uses fewer aircraft.  Or maybe the schedule was built with one or two aircraft available as spares that day.
Depending on a variety of factors, spares may or may not be available, or even deliberately built into the schedule at all.  Having an A-380 sitting around unused all day is an EXPENSIVE insurance policy against something unforeseen happening, but cancelling a flight on something that big is an expensive event when it happens!  If you operate all out of one giant hub and you know that your fleet of 500 aircraft (flying 450 lines every day) is likely to have 2-4 aircraft every day go to the hanger for unexpected maintenance, it's probably a good financial decision to allocate two or three of those lines as dedicated spares.  If your aircraft operate a point-to-point schedule, it's harder (or unfeasible) to keep a spare aircraft at every airport you serve, "just in case" something happens to break there.
There are complicated decisions that airlines will continuously evaluate to find what balance works best for their particular operation.  In some cases, it may be possible to substitute an aircraft from a different fleet for a broken jet... an MD80 goes to the hanger, and a 737 with a reserve crew flies the rest of the day that the MD80 would have flown.  Or maybe a 757 broke, and the decision is to use the 737 to cover the flight, and re-accommodate the extra passengers that the 757 could hold that the 737 now can't.  That's a tactical decision that the airline's operations center will make, considering the availability of both aircraft and crews, as well as the passenger loads.  (They also may pull a "healthy" 757 off of its less full schedule to fly in place of the broke 757, use a 737 that wasn't going to be very full to cover that 757's route, put an RJ on the 737's route that day, and cancel the RJ's flights "due to maintenance."  And if that inconveniences 4 flights of 40-50 people who mostly just get delayed a few hours, that's probably a better plan than stranding two flights of 200 people who'd otherwise have no way to get where they're going until tomorrow or later.)

And one other option. If you're a small airline, and only have a handful of aircraft, for the longer checks or unexpected happens, you might end up using a charter airline to backfill your capacity for the down aircraft. (Both Finnair and Norwegian have used this in the past few years when there's been delays in New aircraft coming(A350 Finnair), or when their out for maintenance(787 @ Norwegian))

To throw more wrenches into the works, the same problems occur with crews. If you're a small airline and one of your pilots is out sick do you have another pilot certified on that particular ac? Is there one available, but he's half way across the country? If weather delays occur the crew can run out of flight hours. You can't just have extra crew sitting around doing nothing.

@TomMcW That is actually worth its own question -- lots of factors in that question, many of them different from working with aircraft.

I think someone did ask that question a while back. I wanted to link to it but I couldn't find it

In general, airlines plan the checks required for aircraft into their schedule. For example, if you have 10 aircraft, you can schedule the routes for only 8 aircraft, assuming that the remaining are in some form of scheduled checks. The bigger problem for airlines is when something unexpected happens, like there is a technical problem or an aircraft is unable to reach its source airport due to weather etc.
In such cases, the airliner can pull a  spare plane available (usually only in cases of large airlines) or 'roll' the next plane available (from a later time slot, usually in another route), hoping that the aircraft will become serviceable in the meantime.

As others have mentioned, you don't use all your airplanes.  You should plan to have some percentage of the fleet in heavy, scheduled maintenance and some other percentage of the feet as hot spares.  The purpose of the hot spares is to cover unscheduled maintenance as it happens and to avoid short delays when a simple 1-2 hour fix is needed.  The remainder of your fleet is what you use to build your schedule.  Yes, this isn't optimal since a fraction of your fleet is never generating revenue, but that is how it works.

Same way you manage anything: with contingencies.
When your car insurance is up for renewal each year, do you suddenly go "oh crap I don't have enough money for this", or do you save it up over the year because you know it's coming?

Hey hi.. Hope you are enjoying stack aviation.. Not sure if you still remember that unawarded bounty!

@anshabhi: Hmm I do not?!


text:
Does a "take-off" assist make sense for electric aircraft?
There is already a good answer that talks about how a catapult provides virtually no benefit for fuel-based aircraft.
However, I'm wondering if the same answer is true for electric planes?  For example, I see here:

The high-lift [leading edge] propellers are all used during takeoff….
  Once you get to a comfortable margin, say 30 percent above stall
  speed, then it will just run off the cruise [wingtip] props and you
  can fold the [blades on the] lift propellers.

This suggests to me that the aerodynamics, complexity, and weight of (at least that design of) electric aircraft could be improved if there were some other means of getting planes up to speed.
While a catapult is a pleasingly high tech solution, there are a number of reasons (some mentioned in that other answer) that make it impractical.
But what about some lower-tech options?  A truck with a tow rope?  A specially designed cart that the plane sits in that accelerates it up to take off velocity?
I know, that's not going to be practical for jumbo-jet sized craft.  But for planes that seat (say) a dozen passengers, could this be a part of the solution?  Or does the math work out essentially the same?

EDIT in response to GdD's question What problem are you trying to solve?
The biggest obstacles to adopting electric aircraft are the limitations imposed by today's battery technology.  My question is intended to determine whether take off assist could address that limitation both by alleviating the high demand for power at take off, and by reducing the weight, complexity, etc of equipment that is only needed at take off.
So, the first part of the question is: Would any type of "take off assist" make useful contributions to the operation of electric airplanes?
Peter Kämpf's answer is "no" for existing aircraft.  But does electric make things different?  Especially given that electric airplanes are currently being designed from scratch?
The second part of the question is: Would any useful "take off assist" for electric planes be viable?
While a steam-powered, extended-length catapult might be both possible and beneficial (and cool), it's hard to believe it's ever going to be practical.  But if a truck with a tow rope allows you to extend the plane's range by 25% while also reducing cost and maintenance, that seems interesting.
To address jamesqf's other comment, my target family of aircraft is something along the lines of Eviation's Alice.
I guess I'm just wondering if electric airplane designers are failing to consider it just because that's how existing planes work?  Or have they done the math?

I think there are already drones that takes off from the top of a running truck. Also smaller ones that takes off by throwing by hand.

A big problem is that you would need such an assist system (and presumably people to operate it) at every place you wanted to take off from - small strips, dirt & grass fields, the occasional dry lake bed...

What problem are you trying to solve? Runway distance? The need for an extra propeller? What benefits do you expect?

One thing worth noting is that the electric engines consume no power while stopped on the taxiway, etc.

A catapult launch makes proportionally more sense for electric aircraft. Proportionally to their lower range and speed, that is.
With current battery technology, only short flights at low speed are feasible. The best one can hope for is to fly a GA-type aircraft for maybe 250 nm at 90 KTAS if the batteries weigh ⅓ of the whole airplane. For this, @Finbar Sheehy's answer assumes an energy density of 250 Wh/kg of the batteries and a discharge to 75% of capacity so we don't abuse the battery.
Now lets further assume the launch will provide the aircraft with all the acceleration to cruise speed, which is 46.3 m/s in metric units. How much of the whole energy used, which is 0.75$\cdot$250/3 = 62.5 Wh per kg of aircraft mass, is that? The kinetic energy the launch provides is 0.5$\cdot$46.3² = 1072 Ws per kg or 0.2977 Wh per kg of aircraft mass. This works out to 0.0047637556 or 0.476% of the trip energy.
That is already quite a bit better than the 0.1385% which were the calculated savings for an airliner, but again not enough to make the case for a catapult compelling. Using a catapult would only save as much energy as covering 1.19 nm at 90 knots. This could become 2 nm if the propellers are used as windmills and the motor as a generator, if the batteries were not full already at takeoff. The most efficient motors, however, make poor generators.
As far as possible savings in installed power are concerned: Electric motors offer much bigger multiples over their rated power for short-time operation - this is how Tesla's "ludicrous mode" is possible, after all. Therefore, it is very unlikely that big savings on the engine are possible. The same, by the way, goes already for airliners: They, too, cruise at or near their maximum rated power in order to fly as high as they do and will not be able to use smaller engines when launched by catapult. The only savings will be in the length of runways – if we find a way to reduce the runway requirement for landings, that is.

If vertical take-off like in the GL-10 demonstrator (to which the question links) is desired, the installed power must be approximately 3 times larger than what horizontal take-off requires. Now the design needs to add propellers and engines which will be deadweight for the rest of the flight - except when a vertical landing is also desired. It makes sense to have both (vertical take-off and landing), so using a catapult will not avoid the additional propellers and engines in most practical cases. Using a catapult will add extreme accelerations if the take-off distance is to be kept short. This will add inertial loads which require a strengthened structure, and again we will get a trade-off which will cut the possible savings to insignificant numbers.
When the V-1 buzz bomb was being developed, the first dozens of launches resulted in many crashes. Since the V-1 was unmanned and the test articles were destroyed in the crashes, it was exceedingly difficult to find out what happened with the primitive instrumentation and telemetry 70 years ago. In the end, it was found that the acceleration on the steam-powered ramp caused the wing attachments to deform and the V-1 became uncontrollable once it left the ramp. Subjecting human passengers to such loads will not be practical, so a catapult does not really help when a near-vertical take-off is desired.

Would there be a real benefit (let's say > 5% range boost) if the catapult could also supply enough power to run the plane's motors at ludicrous levels during the TO run? I am thinking getting to quite a bit greater than just VR while still "plugged in".

@user1008090: The most sensible thing to do would be to run the motor as a generator while the propellers are driven by the airflow. But that will not even add the energy needed for acceleration, and the most efficient motors make poor generators.

Oh well, so much for that patent idea. Thanks.

I'm struggling with 2 apparently contradictory bits of data here.  Your answer (which makes sense to me) says that the amount of energy from takeoff is so small (relative to the entire flight) that there really isn't much point.  But I contrast that with the design in the Q that says they are using high-lift propellers (solely) during takeoff showing that takeoff is going to require a lot more energy than normal flight.

@DavidWohlferd: You are right, my answer only is for horizontal take-off. I need to add something about vertical take-off, too, but VTOL cannot be achieved with a catapult.

I'm not sure they're talking about VTOL.  It looks more like they are just being used to blow (more) wind over the wing to produce lift.

@DavidWohlferd: The high-lift propellers are only there to produce lift when the wing of the GL-10 is tilted, and that is only to achieve vertical take-off and landing capability. If the question is about the power for those propellers, then it must be talking about VTOL.

Yes it would, for fuel based aircraft and for electrical ones. The higher the MTOW, the larger the benefit.
This article makes a case for installing catapults on runways for commercial aircraft: the engines are dimensioned for the take-off, and an assisted takeoff means lighter engines with the associated fuel burn savings. Take-off is particularly demanding on the power installation because it needs to acquire a lifting velocity from standstill.
Other power-hungry flight stages:

TOGA for go-arounds is applied when the aircraft is at approach speed. The more the better of course - but why should GA power be max TO power for the highest payload ever, on the hottest day ever, on the shortest runway ever. This is what the engines are dimensioned for. From the wikipedia article:

..the engines then increase to their computed take off power. Modern aircraft flight management computers will determine the power needed by the engines to take off, based on a number of factors such as runway length, wind speed, temperature, and most importantly the weight of the aircraft.

Climb power is set after TO: a lower thrust/power setting is selected after the TO. When reaching cruise altitude the thrust/power setting is reduced further. So the engines would be dimensioned for the climb stage.

Aeroplanes certified under 14CFR part 25 must be able to climb after a single engine failure after $V_1$: twin engined aircraft will be dimensioned for this case, if the launching device accelerates the aircraft to a high velocity such as $V_4$ there will still be an engine size saving.
A steam catapult will do the trick, or any of the options that OP states. Of course, the take-off run should not be the couple of metres that an aircraft carrier allows, but should be longer to limit the accelerations that the structure is subjected to.

Everybody proposing catapults for launching commercial airplanes has cleary never been on a winch-launched glider. It is brutal.

I'm proposing a REALLY SLOW catapult.

.... nor taken enough psychics classes.
Jokes aside, there is an answer here providing a little math that shows why catapults are, even not considering technical difficulties, not something really useful. Sorry I cannot find it :( maybe someone will, it involved calculating the energy expended for takeoff of a 747 vs the total energy required in flight :-/

@Caterpillaraoz That answer is linked in the OP. While the math is correct for a given aircraft configuration, it does not consider weight savings from smaller engines. The other answers there mention many problems that an aircraft carrier catapult would bring. While also true, that is obviously not the optimal catapult or assisted take-off configuration for a commercial airliner take-off.

Smaller engines won't help much - the MTOW of a 737-600 is 144,500lb, using two CFM56  engines that weigh 5,216 lb each. By the time you reinforce the plane to handle the catapult, your weight savings will be minimal. (all weights taken from Wikipedia)

@Eugene Styer: I don't think the OP is asking about large commercial aircraft - certainly such aren't practical with current tech - but about GA aircraft, electric versions of which do exist.

@Koyovis, engines are usually fully utilized throughout the flight. The TO/GA power is time-limited. The climb power is all that the engine can provide for extended time without impractical wear and since for efficiency you generally want to climb as high as you can, the reduction from climb to cruise is quite small. There would be some difference for electric engines as they don't lose power with altitude like air-breathing heat engines do though.

For another aspect, anything that is not able to go around under its own power stands about zero chance of being approved for commercial operation with passengers. Small GA planes might be though—after all, gliders can't go around.

@EugeneStyer You don't need to re-inforce the plane to handle the catapult. Limit the acceleration of the catapult to what existing landing gears can handle, that is a lot already. Their limit load is a vertical speed of 10 m/s, that is a huge vertical and horizontal impact. Also, weight savings works at any weight. You set it against MTOW, but at any point in the cruise reduced weight causes reduced induced drag. That adds up over the lifetime of an aircraft.

@JanHudec Yes good point, air breathing engines are dimensionsed larger, then de-rated to provide continuous climb performance.


text:
What would be the advantages and drawbacks of a single-propeller tiltrotor?
When looking at existing tiltrotors, most of the designs use two propellers, sometimes four. helicopters (and light planes), on the other hand, generally use only one propeller.
Have there ever been designs or proposals using a single rotor (or contra-rotative rotors on the same axis) instead? If so, what advantages and drawbacks did they have? If not, are there specific reasons why this configuration is always ignored?
The most obvious one would be that the engine would sit right on the cockpit, taking its place or at least blocking visibility. But we can imagine a tiltrotor UAV where this would not be an issue.
For the needs of this question, tail-sitters like the Convair XFY Pogo are not considered tiltrotors, as the entire craft changed orientation instead of only the engines.

How is this different from a helicopter and how would it counter torque?

I feel the current phrasing invites opinion-based answers

@AEhere The difference with a helicopter is that it could switch to horizontal flight, presumably using wings. Countering torque may be a significant drawback, and explaining why it would be difficult would probably have its place in an answer.

@Federico How should I rephrase it to make it less opinion-based?

@Eth I agree with Federico's point here because as it stands you are vaguely describing a non-existent aircraft and asking what the pros and cons would be in operation (never mind development), without defining the design to be at least believable. Therefore, each respondent can make their own interpretation of how it should look (counter-rotating helicopter with tilting? vectored thrust? with or without cyclic control on the rotor?)

@AEhere I rewrote the question, hopefully it should be less opinion-based now

Such an aircraft can't exist.
Helicopters don't work with a single rotor, they need at least two of them to counter torque. There are several popular designs, including conventional, coaxial, intermeshing and tandem rotors.
All tiltrotors operate as tandems in helicopter mode. It's the only setup where both rotors can be placed at the nose, tail or wings. In all other designs, at least one of the rotors needs to be mounted in the middle, above the center of gravity, where it can't be tilted without crashing it into something.

So it is impossible to use another scheme than tandem rotors for a tiltrotor? Also, at which point does a plane rotor becomes too big and needs to counter torque?

@Eth - Lift needs to be aligned with the center of gravity, otherwise the craft will tip over. If you can't mount a rotor in the middle, the only remaining option is to mount two or more rotors evenly spaced around the center - a tandem, quad- or hexacopter is born. Also, all rotors need counter torque, but in fixed wing aircraft there's rudder and ailerons for that.

At which point does the propeller becomes too big for an aircraft to counter its torque? I suppose such tiltrotor could use tip-jets, counter-rotating propellers or maybe NOTAR, but it doesn't seem worth it.

@Eth - You still have the problem that a single propeller has to be mounted in the middle, where it can't be tilted.


text:
Do rotating deflagration engines exist?
Pulsejets (which could have been called pulse deflagration engines) rely on pulsed deflagration to generate energy and thrust. Their pulsed mode makes them hideously loud and rather inefficient, but they can be quite simple to build - they can even have no moving parts.
Pulse detonation engines work in a rather similar way, but rely on pulsed detonation instead of deflagration. It makes them much harder to build (and possibly even louder), but theoretically much more efficient.
Rotating detonation engines are a further development. The pulsed regime being source of inefficiencies, vibrations and noise (so much noise), it is replaced by continuous detonation fronts endlessly going around an annular chamber, with fuel and oxidiser injected behind each front, for the next one.
Is there such a thing as a rotating deflagration engine? Is it theoretically feasible?
The advantage appears obvious at first glance: avoid the inefficiency of pulsed mode from a pulsejet, while avoiding the complexity of using detonation for more manageable (if less efficient) deflagrations. One could also hope to keep some of the simplicity of pulsejets, with fewer moving parts and air being aspired in the chamber as part of the cycle.
However, I couldn't find any reference to such an engine, whether attempts to build it, studies, or even description of why it wouldn't actually work.
Note: this is not about turbines or rocket engines. Those are technically continuous deflagration engines, but there is continuous pumping of fuel and oxidiser (air from the compressor in the case of the turbine), and a stationary burn zone. A rotating deflagration engine would have moving deflagration fronts with fuel and oxidizer (like air) injected after a deflagration front moves away.

Isn't a common gas turbine a continuous deflagration engine?  The flame fronts don't rotate, though, nor in a ramjet.

@ZeissIkon Like the rocket, it is a continuous deflagration engine, but not a rotating deflagration engine as meant here: you are pumping fuel and oxidizer (air from the turbine) and have them burn stationary, instead of having a front circling an annular chamber, with injection right after it has passed. Edited the question to add the precision.

How would you extract the energy from the hot gas without pistons or a turbine?

@ZeissIkon The pulsejet (especially valveless) is an interesting example for that. According to the explanations there, the air itself acts as a piston. That said, rotating detonation engines can use a compressor for air intake and be used in a jet engine (or alternatively as a rocket with both stored fuel and oxidizer), so it is not incompatible. Of course, it would be more interesting if it was possible to design a rotating deflagration engine without a turbine (or even with no moving part, while we're at it), but it may not be possible.


text:
Did WW2 Bombers have shock-absorbed pilot seats?
I'm trying to figure out if the cockpit seats for WW2 bombers had any kind of shock-absorption, or were they fixed rigidly to the airframe?
This picture is from a B-24. There are clearly enough pivot points and sliders there to allow for movement, but is that movement just to allow different pilots to adjust the seat, or is that shock-absorption?

Similarly with this picture from a B-17, there is enough mechanical stuff connected to the back of the seat, but I haven't been able to get a good enough picture to actually determine what it's for.

If these planes did have shock-absorbing seats, does the whole seat bounce around as a unit? Or does the backrest move independently from the base of the seat, like on some reclining chairs?
Though not a bomber, here is a seat from a Spitfire. It just looks so much like it has mechanical shock absorbers. The seat is attached at the top and the bottom by rods that are hinged at each end, and then there are diagonally connected pistons. What else would those possibly be for?

Most had cushions, which is a type of shock-absorber.

No @PartyArk, in warplanes of that era you sat on your parachute, that's why the bottom of the seat looks like a pan.

I'm fairly confident that even passenger seats (at least in cargo, err coach class and business) don't have shock-absorption either, even today.

Yeah I guess it makes sense, but like look at this blueprint for a Spitfire seat? What are those pistons attaching the seat to the back of the cockpit if not shock-absorption? https://hosting.photobucket.com/albums/ff321/taylortony/Aviation/.highres/seat2.jpg

No warplane seat I've ever seen has any kind of shock absorbers, if you mean springs or hydraulics. You don't want your seat bouncing around when you're trying to control the airplane. The frames are rigid, allowing for adjustments.
If you are asking about cushioning, the seat backs usually had something for comfort, warplane pilots almost always sat on their parachute, using that as a cushion.

As @GdD says, if you mean mechanical shock absorbers then no.
In WW2, aircrew used cushions to absorb and damp shock impulses.  The pan of the B-24 seat in your top picture accommodates a cushion alone, and is typical of warplane seats of the era. Your second picture is a Spitfire seat: the deeper pan would accommodate a parachute underneath a cushion, a less common arrangement.
(The piston behind the Spitfire seat is to aid with the adjustment of the seat height, not shock-absorption. Spitfire pilots would raise their seat while taxi-ing to aid visibility.)
There is some confusion about pilots and aircrew using their parachutes as a makeshift cushion which I will attempt to clear up. A packed parachute is very dense, and makes for an appalling shock-absorber.
First, to fighters. In order to expedite egress from the cramped confines of a small fighter cockpit it was necessary for the pilot's chute to sit underneath him or her in the seat pan. In this arrangement, a seat cushion was placed between the parachute and the pilot's bottom as part of the assembly. The giveaway for these cushions is that they tend to have a cut-out for the rigging to pass through. The RAF, Luftwaffe and USAAF all used this configuration for small cockpit scenarios.

Cadet L. Deitz at the Naval Air Base, Corpus Christi stands on his plane in August 1942. source
 F/O Gordon Hill with his MkXVI Spitfire in 1945 5]5
As the war progressed, back chutes became more popular. For instance, 80.3% of bailouts from a P-47 were done with seat chutes, but 96% of P-51 bailouts were done with back chutes. The extra space underneath you meant more room for emergency equipment and the possibility of a more substantial cushion that could be used as a buoyancy aid.
But your question was about bombers, and here the picture is a little more complex, although the answer is still the same.
In neither the RAF nor the USAAF did bomber aircrew typically wear a parachute. Every station had a stowage area for a clip-on chest parachute - for the pilot(s) of the B-17 and Avro Lancaster, it's behind their seat. If you have to move around - and most aircrew did - wearing a solid and heavy parachute would be impractical. And if you don't (tail gunner, ball-turret gunner), there's not enough room to wear one even if you wanted to.
 Original U.S. WWII Boeing B-17 Flying Fortress Cockpit Seat Cushions source
All crew members would wear a harness (the Observer Harness in the RAF) and attach a clip-on chest parachute in the case of emergencies, stowed near their station. It's almost inconceivable how stressful finding your parachute in a real emergency must have been. The pan you see in your seat is for a cushion - most USAAF bombers (including the B-24 in your picture) would have bright yellow cushions marked "Do not remove from airplane" in the shallow pan of the seat at their various stations - you can actually see them in the picture of the B-17 you have provided.
 USAAF A-3 Harness and clip-on waist pack source
For the B-24 and B-17 we have stats which tell us only a small minority of bailouts were done with seat chutes, the rest with mostly chest and sometimes back chutes.
 B-17 crew stations showing standard issue cushions source
The Lancaster pilot was an exception: while he was provided with space behind him to stow his chute, as the war progressed increasingly the seat chute harness was more popular. Hence his cushion was in-between the chute and his bottom, like a fighter pilot: with only one pilot, he wasn't expecting to leave his seat for any other reason than bailing out.

WW2 B-29 cockpit show comfortably-cushioned seat. source
So in answer to your question, B-24 aircrew, as with most WW2 aircrew, primarily used cushions to absorb some of the energy of the aircraft. They did not use their parachutes as cushions.


text:
Is corrected astigmatism disqualifying for an EASA Class 1 medical?
I realize there are similar questions on the site already, but not one specifically dealt with my problem. I reside and plan to work in Europe so I looked up the EASA regulations, but I couldn't really make sense of them. My question is: I have astigmatism, which is corrected by my glasses. I am also shortsighted, which is also corrected. Does this disqualify me for a Class 1 license?

I have a registered astigmatism on one eye that is balanced by the other eye. I have a Class 1 medical w.o. problems. In my medical record, the Doc wrote that my astigmatism corresponds to a .75 dpt on one eye.
He also told me that absolute values are not important, important is that the astigmatism is corrected by either the other eye, some glasses or it is so low that the brain can even correct for it.


text:
Why does the B-29 not have pilot-accessible thrust?
Watching this incredible B-29 Superfortress flight, I was amazed to see that it required a dedicated person to manage the 4 engines and their power levels.
Why was the throttle not placed between the two pilots, like the modern airliners of today?

@Sanchises "Why was the throttle not placed between the two pilots, like the modern airliners of today?"  I read this as asking why the pilots didn't have a common throttle lever set between their stations -- which I covered in my answer.

@ZeissIkon I see - interesting answer!

Well worth the watch if you have the time: https://www.youtube.com/watch?v=R5D1f_1XU8w

It looks like it also requires a dedicated person to make a video of the flight through the nose glass...

This setup was actually pretty common for four-engine aircraft of the day.  Look at the cockpits of airliners from DC-4 to Lockheed Constellation, and you'll always see a dedicated flight engineer station -- not to the level of managing throttles (which would require a voice command from the PIC to change power setting, impractical for the reaction times required in flight), but to manage the relatively complex and failure-prone large radial engines.  The flight engineer station persisted well into the jet era -- at least to the early versions of the 747, which first flew in 1969.
However, if you look closely at a B-29 cockpit, you'll see each pilot has an individual throttle lever set -- they're not between the yokes because of the crawlway for the bombardier station.  Rather, they're outboard of the seats, near the (presumably pitch) trim wheels.  There were also propeller controls, though they're harder to identify in a cockpit photo.  With constant speed propellers, the only control required on an immediate basis was throttle (the flight engineer managed synchronization), and both control seats had throttles, as well as the ability to feather a dead engine.

The pilot had propeller controls as well.  4 toggle switches with a gang bar for simultaneous operation of the Curtis Electric propellers.

Thanks, @JohnK -- edited.

What would happen if the pilots moved their throttle levers in opposing directions?

@Sean the controls were tied to the same cables, so if one was moved, the other moved as well, same as the yokes and rudder pedals.


text:
Why isn't the V flight formation being used for commercial purposes?
The V Flight Formationhas been found to be helpful in achieving greater fuel efficiency and range. Though it is currently used mostly for military purposes, what restricts it from being used for commercial purposes?

Quite a lot of time and effort ( and money ) has been spent, and is being spent now, on keeping large commercial aircraft away from each other....

This would have been my solution to the quest for lowering flight cost: Use two A330s, one commanded by a human pilot and occupied by the passengers, and the second one flown by the computer in close formation, loaded with the baggage and freight. This could achieve the seat mile cost reduction of the A380 without the development expense.

I think another interesting, and related questions might be "Why do airlines operate one big airplane on a scheduled flight rather than several smaller ones."  Because really that's where formation flying would come into play, if that scenario were to exist.  I mean, if you have a few planes that could fly in formation to another place...why not just put them all in one much bigger airplane?

lol, I guess @PeterKämpf is saying basically the same thing.  Though rather than split the passengers from their baggage, I'm suggesting just having two flights to begin with.  Which might be nice, actually, in terms of flexibility...

It may not be efficient for commercial purposes but it's a great way to win a hockey game.

This possibility has actually been studied and discussed (e.g. here, here, here) as a way to save fuel costs, but there are several issues with implementing it today:

You need to schedule a number of aircraft all flying the same route at the same time for a significant amount of time
If different, competing airlines should participate in one formation then they have to cooperate and be satisfied that no one is at a commercial (dis)advantage
A lot of work would need to be done on procedures and training (how does the formation form up, who takes the lead, how/when does the formation leader change, how are emergencies handled, what flight levels are used, what separation is required between formation and non-formation flights etc.)
Autopilots, software and other equipment would need major updates, which in turn requires re-certification
All relevant regulations on separation, formation flying and so on would need to be reviewed and changed which in most countries means changing the aviation laws, and changing any law is usually a long, slow process

Having said all that, you'll notice that most of those issues are procedural, not technical. In theory it's a viable approach, but turning that theory into reality would require a lot of work.

Bullet three is important. The lead goose in a V changes very often. If you don't swap out the lead, then there is bullet two to consider.

Looks like geese are far superior to humans in cooperative organization. :-)

Knowing very little about flights and the like, I think it would probably involve lots of logistics for landing at an airport, too - don't think they could land at the same time. And maybe that logistics don't cover the savings of the V formation.

Formation flight is not allowed by the FAA when carrying paying passengers:
from 14 CFR 91:

§91.111   Operating near other aircraft.
(a) No person may operate an aircraft so close to another aircraft as
  to create a collision hazard.
(b) No person may operate an aircraft in formation flight except by
  arrangement with the pilot in command of each aircraft in the
  formation.
(c) No person may operate an aircraft, carrying passengers for hire,
  in formation flight.

The core reason would be safety. If any craft has a problem it needs to be able to get out of formation safely and pilots need to be trained for it. Also flying at cruise requires the autopilot; however no current civilian autopilot can make adjustments to account for maintaining separation in formation flight.

This is quite specific to passenger aircraft. Are there any similar regulations or logistical reasons against allowing this for cargo carriers? (I suppose the autopilot limitation could be one.)

As a passenger I wish:

To have a good choice of flight times
Not to have to wait a long time while other aircraft are unloading
Not to have long queues
Not to have my flight be delayed due to issues with another flight

The above does not sit well with 'You need to schedule a number of aircraft all flying the same route at the same time for a significant amount of time'.